External pipe thread through rolling has attracted attention owing to having higher mechanical connection strength and excellent sealing performance compared with external pipe thread through cutting. The corresponding manufacturing processes have also been disclosed in Chinese patent CN200310111695.2 and No. CN200710106912.7. However, according to these two processes for rolling an external pipe thread disclosed in the above two patents, a conical surface is achieved through axial punching or radial extrusion prior to rolling an external pipe thread. As compared with one-off shaping process for cutting external pipe thread, the two processes require that a hollowed cylindrical blank is axial punched or radial extruded with a conical mold mechanically or hydraulically, i.e. a conical surface is processed firstly, and then a external pipe thread is processed on the conical surface which have been processed.
Obviously, these two processes for rolling external pipe thread have the following three problems:
1. As compared with existing external pipe thread process by means of threading and cutting, the process for rolling an external pipe thread needs an additional conical surface processing procedure, it is time-consuming and inconvenient for on-the-spot external pipe thread processing for the pipe network, thus unacceptable.
2. Due to extremely high pressure produced by instantaneous axial punching or radial extrusion, piping material at the junction between original outer diameter of the steel pipe and the conical surface, especially weld on the welded pipe, is vulnerable to hidden and apparent damages to the extent of incurring safety hazards to external pipe thread products when shaping conical surface.
3. Outer diameter of existing rolled steel pipe blank defined by a cutting process is excessive for rolling. Although taper of conical surface formed through punching is 1:16, taper of conical surface on inner diameter of rolled products is over the standard taper of 1:16, or length of the conical surface greatly exceeds the permissible standard value. As a result, the actual maximum reduction in inner hole on the rolled external pipe thread product exceeds the permissible maximum reduction in inner diameter as incurred by accumulated maximum discrepancy to wall thickness of steel pipe and outer diameter of the steel pipe. This may affect fluid conveying stability to some extent.
On the other hand, failure to proceed with conical surface processing to the outer diameter of existing standard steel pipe may result in either excessive dimension of external pipe thread rolled or deformation or cracking to the pipe. Therefore, punching or extrusion is an essential process for external pipe thread rolling. This is mainly caused by defects of processes designs in the above two patents and outer diameter of steel pipe used as rolling for an external pipe thread in the technology of cutting processes for rolled steel pipe.
In view of the fact that excessive outer diameter of milled steel pipe is inappropriate for direct rolling, it is applicable to proceed with rolling to reduce outer diameter of the steel pipe to the designated dimension, namely standard intermediate diameter for external pipe thread based on rolling techniques prior to rolling of conical external pipe thread. However, such method for reducing of outer diameter of steel pipe to be rolled may result in such problems as difficulty in controlling ovality, damage to the plated coating on the surface of steel pipe and reduced inner hole on the steel pipe after rolling. The method to solve such problems is to directly produce rolled steel pipe of special outer diameter lower than that of the standard steel pipe according to technical requirements for rolling. However, as there are no national or international standards on outer diameter and wall thickness of rolled steel pipes, it will take a relatively long time for market promotion and acceptance. This may inevitably affect the promotion and application of external pipe thread rolling processes.
In view of aforesaid description and analysis, it can be seen that, for current steel pipes having a standard outer diameter, omission of axial punching or radial extrusion and simplification of technical procedures for the purpose of producing rolled external pipe thread according to national and international standard and people's operation habit are critical to promote a perfect external pipe thread rolling technique.